A capacitor cell typically comprises an anode having a dielectric layer, a separator, a cathode, and an electrolyte solution. The anode and cathode often comprise stacked or coiled metallic foil members although pressed, sintered and formed powdered metal anodes are known and used in the art. The energy of a capacitor cell is stored in the electromagnetic field generated by opposing electrical charges separated by the dielectric layer disposed on the surface of the anode. Etching may be used to increase the surface area of the anode, as the energy stored by the cell is proportional to the surface area of the anode. A dielectric oxide layer is formed on the anode when a voltage is applied in an electrolytic solution. The dielectric layer insulates the anode from the cathodic electrolytic solution, allowing charge to accumulate. The separator holds the anode and cathode foils or powdered slug-type anodes apart to maintain charge and prevent short-circuiting. In one embodiment, the anode/separator/cathode laminate is typically rolled up to form a cylindrical coiled member and encased, with the aid of suitable insulation, in an aluminum tube that is subsequently sealed with rubber material. In such embodiments it is imperative that the cathode foil and anode foil be precisely positioned opposite each other on the separator material and be adequately separated by this same material.
An alternative design, commonly used in implantable cardioverter-defibrillators (ICDs), are flat (e.g., stacked electrode), compact aluminum electrolytic capacitors. These flat capacitors have been developed to overcome some disadvantages inherent in commercial cylindrical capacitors. For example, U.S. Pat. No. 5,131,388 to Pless et. al. discloses a relatively volumetrically efficient flat capacitor having a plurality of planar layers arranged in a stack. Each layer contains an anode layer, a cathode layer and means for separating the anode layers and cathode layers (such as paper). The anode layers and the cathode layers are generally comprised of foil plates of anode or cathode material and are usually electrically connected in parallel. In a paper “High Energy Density Capacitors for Implantable Defibrillators” presented at CARTS 96: 16th Capacitor and Resistor Technology Symposium, Mar. 11-15, 1996, several improvements in the design of flat aluminum electrolytic capacitors are described, such as the use of an embedded anode layer tab and solid adhesive electrolyte. Further advances in flat electrolytic capacitors are found in U.S. Pat. No. 6,006,133, issued to Lessar et al., which is incorporated by reference.
For flat, powdered metal, or cylindrical capacitor cells, it is necessary that the anode and cathode remain separated. A minimum separation between the anode and cathode must be maintained to prevent arcing between the anode and cathode, and to allow charge to accumulate without short-circuiting. In cylindrical cells, the anode and cathode foils are aligned precisely with a separator positioned between them and coiled tightly to prevent movement of the anode, cathode and separator during subsequent processing and use. Spacing is typically maintained at the electrode edges as well by providing separator overhang at the top and bottom of the anode and cathode winding, to prevent short-circuiting to the casing. In flat capacitor cells, anode to cathode alignment is typically maintained through the use of internal alignment posts or screws (as described, for example, in U.S. Pat. No. 6,006,133 to Lessar et al.). Alignment of the anode and cathode plates in flat capacitor cells again can be somewhat problematic in that the plates are generally small and difficult to maneuver and maintain in position during assembly of the capacitor cell.
Maintaining a proper distance between cell components is thus one of the prime functions of a separator. A separator must be resistant to degradation, have sufficient thickness to maintain inter-electrode separation without interfering with cell high performance, and exhibit sufficient surface energy such that electrolyte wettability and absorption are augmented. The enhancement of the wettability and absorption properties is desired since such enhancement is likely to reduce the equivalent series resistance of the cell thereby increasing the fraction of the stored energy delivered to the medical device. However, the separator must also have an electrical resistivity sufficiently high to prohibit short circuit current from flowing directly between the electrodes through the separator and tortuosity to provide adequate ionic transfer. These requirements are balanced by the need for the separator to have porosity sufficient to maintain electrode separation while allowing ionic transfer to occur unimpeded within the electrolyte during discharge. Additionally, the separator must have sufficiently strong tensile properties to facilitate cell fabrication and to withstand internal cell stresses due to changes in electrode volume during charge/discharge cycles.
Separators are generally made from a roll or sheet of separator material, and a variety of separator materials have been found to be effective. Paper, particularly Kraft paper, is a cellulose-based separator material that is commonly used. Cellulose separator materials are manufactured with high chemical purity. The total thickness of cellulose separators employed between anode and cathode plates will vary with the voltage rating of the capacitor structure and the type of electrolyte employed but, on the average, the thickness varies from 0.003″ to 0.008″ in connection with capacitors rated at from 6 volts to 600 volts.
A common alternative to paper separators are polymeric separators. Generally, polymeric separators are either made of microporous films or polymeric fabric. An example of a microporous film separator is a separator comprising polytetrafluoroethylene, disclosed in U.S. Pat. No. 3,661,645 to Strier et al. U.S. Pat. No. 5,415,959 to Pyszeczek et al., on the other hand, describes the use of woven synthetic halogenated polymers as capacitor separators. The use of “hybrid” separators comprising polymer and paper material has also been described. See, for example, U.S. Pat. No. 4,480,290 to Constanti et al., which describes the use of separators including a porous polymer film made from polypropylene or polyester and absorbent paper.
In the assembly of a capacitor cell it is important to maintain orientation, contact and, as applicable, alignment of the anode, cathode, and separator components. Failure to align these components may lead to short-circuiting or inefficient capacitor performance. For example, in cylindrical capacitors, proper spacing is typically maintained at the electrode edges or peripheries by providing separator overhang at the top and bottom of the anode and cathode winding, which results in a larger capacitor than would otherwise be necessary. In addition, the anode and cathode are precisely aligned and coiled tightly by a winding machine to prevent movement of the anode, cathode, and separator during subsequent processing and use. Alternatively, in flat capacitors, anode to cathode alignment is typically maintained through the use of internal alignment posts. Build-up of static charge in the separator material during manufacture of such capacitors can make handling of the material particularly troublesome. All of these techniques have the disadvantage or requiring extra machinery or capacitor components that would not otherwise be required.
It would be desirable to employ in capacitor cells, such as batteries or capacitors, a separator material that is sufficiently thin, has strong tensile properties, possesses enhanced wettability and absorption characteristics, is resistant to degradation in the cell environment, and has a precise porosity sufficient for use with a particular capacitor cell.